With more and more renewable plants feeding South Africa’s electricity grid, battery storage may hold the key to bridging over those moments when the sun doesn’t shine or the wind doesn’t blow. But for now, batteries are imported, and expensive, writes Leonie Joubert.

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If a bank of clouds moves across the sun, above a solar photovoltaic (PV) farm, it’s like… well… flicking off a light switch. In seconds, a 100 megawatt-capacity (MW) plant can throttle down to just 10 MW, according to Bernard Bladergroen, associate professor with the University of the Western Cape’s Energy Storage Initiative Lab. If that happens, you need a fully charged battery to kick in instantly, to keep the grid stable.

As a grid like South Africa’s (SA) becomes more dependent on wind and solar power, it’s going to need batteries on constant standby for moments just like this.

Traditionally, SA’s electricity grid managers have levelled out the peaks and troughs in a few ways. One, is through pumped water storage: build a dam up on a plateau, above a perennial river. During low-demand times of the day, the country’s coal stations will run on full power and the excess energy will drive pumps on those rivers, pushing water up into the dam. Later, when demand for power climbs and the coal stations can’t keep up, the dam releases the water through a small hydro scheme, with gravity doing the work. Because these dams generally last 40 or more years and need little maintenance, they’re a cheap way of storing energy during off-peak times, and releasing it during high demand times.

But, says Bladergroen, it’s a relatively slow system to kick into operation, taking about 15 minutes from switching it on, to getting energy into the grid. If you’re dealing with the sudden and unpredictable variability of solar or wind power, you need a power storage system like a battery to kick in instantly, and hold the grid stable until that pumped storage backup comes online.

In principle, batteries are ideal, and there are several exciting developments in the world of battery technology that will make things easier in future. In particular, says Bladergroen, advances in lithium ion and sodium metal halide batteries provide alternatives to the heavy, less efficient lead acid batteries we’ve used for so long.

Big batteries: sodium metal halide for utility-scale storage

Pumped storage accounts for nearly all of the grid’s stored energy in SA. It can support the utility-scale renewable energy programme under development here. But this sort of grid will still need battery support. At the moment, thought, there aren’t any affordable battery solutions that can handle utility-scale storage loads, particularly with the price of oil and gas so low.

But the sodium metal halide battery, which was developed right here in South Africa back in the 1980s and has since been developed further abroad, is now being revisited as a grid-scale option.

Bladergroen’s institution is working with the national utility, Eskom, and Department of Science and Technology, to bring this technology home. Ideally, sodium batteries the size of shipping containers could one day be installed alongside a solar or wind farm, to kick into action the second the sun disappears or the wind drops. However, even though the materials for this kind of battery are cheap, the current cost of production is high.

Small, local, decentralised: lithium ion batteries

The problem with lead acid batteries – like those in your car – is that they have a short lifespan (typically one to two years, depending on their use), they are heavy (they’re unsuitable for use in electrical cars, for instance), and they deteriorate faster above about 40C (some telecommunication sites here have battery storage units housed in air-conditioned containers, which are cooled permanently in order to prolong battery life).

Advances in the lithium ion battery means this tech is giving the clunky old lead acid battery a run for its money. Even though they’re still more expensive, unit for unit, the lithium ion battery has a much longer life – up to eight years – and can handle many more charge-discharge cycles. They’re also more efficient than lead acid, where you get back about 70% of the energy you put into it when it’s being charged up. With lithium ion, you get 90% of the energy back.

This sort of battery is ideal for daily use, particularly for small-scale and remote off-grid needs, such as small factories or farms.

Bladergroen argues that it could also serve remote towns, where transmission lines aren’t enough to keep up with the few hours of daily peak demand. Installing a larger lithium ion battery at the end of the transmission line to pick up the slack during high demand times would often be more cost effective than upgrading the transmission lines to meet the peak-time shortfall.

The current cost reductions in this technology makes it a suitable candidate to improve the quality of grid power (frequency and volt), not the quantity, says Bladergroen.

Still not ‘home brewed’

The problem is that none of these new battery technologies are made at commercial scale in South Africa. There are some experimental labs working on them, but for the foreseeable future, the sector has to import batteries.

Once these are affordable and widely available, they’ll allow for the kind of grid which Bladergroen argues is possible and desirable: one that’s mostly driven by renewables, with battery support, and the occasional use of gas as a top-up when those can’t quite keep up with demand. Ultimately, though, it’s about weaving together a series of complementary technologies to keep the grid stable and meet the region’s energy needs.